In recent years, there have been demands for high wet grip performance and low rolling resistance in pneumatic tires. A known technique for satisfying these demands is to blend a styrene-butadiene copolymer or a reinforcing filler such as silica into a rubber composition constituting the cap tread of a tire. Further, to enhance the wear resistance, rubber hardness, or modulus of repulsion elasticity of the rubber composition, it has been proposed to blend a polybutadiene or silica having high reactivity, for example, but this is problematic in that the rubber strength or rubber hardness decreases or the workability is diminished.
Japan Unexamined Patent Publication No. 03-239737 discloses that a pneumatic tire in which a rubber composition prepared by blending a styrene-butadiene copolymer having a specific sequence of styrene units and silica is used for a tread simultaneously satisfies wet skid resistance, rolling resistance, and wear resistance. However, this rubber composition is not necessarily capable of sufficiently enhancing wear resistance and rubber hardness, and is not therefore capable of sufficiently satisfying the demands of consumers.
Japan Unexamined Patent Publication No. 57-179212 discloses that an ozone-decomposed product of a styrene-butadiene copolymer is analyzed by gel permeation chromatography (GPC), and the content ratio of a long-chain styrene block and the content ratio of a simple chain with one styrene unit with respect to the total styrene content are measured. A styrene-butadiene copolymer in which the long-chain styrene block content is not greater than 5 wt. % and the simple chain content with one styrene unit is not less than 50 wt. % with respect to the total styrene content in the styrene-butadiene copolymer, and the total styrene content is from 10 to 30% of the styrene-butadiene copolymer is then further disclosed. However, a rubber composition made of this styrene-butadiene copolymer is not necessarily capable of sufficiently enhancing the rubber strength, rubber hardness, wear resistance, and low hysteresis loss thereof. In addition, in the GPC-based analysis in Japan Unexamined Patent Publication No. 57-179212, even in a case where the ozone-decomposed product of the styrene-butadiene copolymer can be classified based on the number of styrene units and a chain made of only styrene units, the number of units derived from 1,2-bonds of butadiene in the ozone-decomposed product cannot be identified. Further, in the GPC-based analysis of Japan Unexamined Patent Publication No. 57-179212, the ratio of the styrene content in the decomposition product to the total styrene content is calculated, and the amount of each decomposition product is not accurately quantified.